Stock TH400 no cooler line flow

[Adam Whitman]

30-year old TH400 with old B&M shift kit and 12" stock GM VP converter that worked fine when pulled out. Pulled the trans 2-years ago and long story short it went back in without so much as the pan coming off. Fired it up, ran it in and out of the shop and down the street a few times shifts good everything seems fine.

Pulled a cooler line to pump the pan down before a filter service and there was nothing except a dribble out the return line. Pulled the line and blew backward through it to verify open lines; all is good.

Serviced the trans filter and reassembled. trans still functions fine, but no cooler flow in P, N, or D, high-stall for sure, I think I checked in low-stall too.

Did a search here and my head hurts from reading about converter pressure, thrust bearings, and cooler line pressure. BUT, didn't find anything about what would kill cooler line flow.

All I could dream up is that the regulator valve is stuck or the front pump is so bad there's no volume left for the cooler line. However all of this talk about converter pressure and the cooler line makes me think I'm not understanding the fluid paths correctly.

Help?

 

[turbobitt]

PR valve could be stuck. With what you described, I would be surprised the converter hasn't pumped itself dry and started to cavitate and stall higher. Converter must be getting hot.

You can try and check line pressure, it should read very high if PR is stuck. The PR valve is accessable when the pan is removed. You can remove the clip ring, boost valve and spring and check the valve for freedom of movement.

Allan G.

 

[Adam Whitman]

OK, I'm glad that you agree with my logic that it might be a stuck PR valve. If not the only thing I can think of is he pump is toast.

I haven't driven it over 3-miles, for this reason so maybe that is why there hasn't been any converter cavitation.

I'll get under it and give my hair an oil job and see what happens. Hope I can get that sucker loose, the last time I did this in the car I spent a whole night trying to get a stuck PR plug out!

Thanks for the reassurance I'm looking in the right direction.

 

[Adam Whitman]

PR valve didn't feel stuck. however, while perusing the Sessions TH400 book I noticed that the VP TH400 had a cooler bypass valve. It bypasses with too much back-pressure from the cooler (obviously not my problem when the cooler is unhooked). I'm thinking the bypass valve might be stuck, but can't check it out without pulling the trans.

Will re-test tomorrow when I can get more ATF from the parts store.

 

[SSedan64]

Did you remove the Bottom line on Trans= Feed Line to Cooler?
If you removed the Top Trans Line/ Return line from Cooler & had no flow, the Cooler could be stopped up.
Just checking.

 

[Adam Whitman]

Actually removed both lines after the bottom line had no flow. The top line would drool while it was running, that just made me realize that if the bypass valve is in fact stuck, maybe that would cause a little bit of back-flow through the return line and hence, the drooling.

I also blew backward through the coolers, which are new, just to make shure there wasn't a slug of crud somewhere that caused plugging.

Interestingly, although I didn't drive too far, I took it out on the road and beat on it a little bit and my infrared thermometer said the pan temp was only about 160. I think I'm going to drive it a bit and monitor the pan temp. And pray that the bypass valve unsticks... it's maybe a 13-sec streetcar, so not a huge load plus I hope the stock VP converter shouldn't generate too much heat in low-stall.

 

[turbobitt]

I don't think I would drive it. The cooler return also serves as the lube oil for the output shaft, bearings and planets.

Allan G.

 

[Adam Whitman]

When the cooler bypass valve is bypassing, it is still supplying oil to the above-mentioned items. Also, with the return line drooling in reverse-flow I think this is indicating that there is lubrication going on there. Also as was mentioned, if the converter wasn't getting oil it would be cavitating; all seems to be good with the converter. Pan temps haven't been found to exceed 170. I'm going to try applying engine vacuum to the cooler line and see if it will suck the bypass open; tried a hand pump and it won't pull enough volume.

Or...I'll kill the damn thing...and be forced to attempt repair of the 200-4r or go with a 4l60E or 80E. Every local trans guy I know runs from the 200-4R.